BE1021907B1 - METHOD AND SYSTEM FOR PROCESSING BIOMASS - Google Patents

Abstract

The present invention relates to an integrated process for the processing of biomass comprising fermenting, drying and combusting the biomass. Also provided herein are methods for the production of biofertilizer and the production of (green) electricity comprising the method according to the invention. The invention also provides a system for processing biomass according to the method according to the invention, as well as bio-fertilizer obtained from the method according to the invention.

Description

METHOD AND SYSTEM FOR PROCESSING BIOMASS TECHNICAL FIELD

The present invention relates to a method and system for processing biomass into bio-fertilizer and optionally electricity, in particular green electricity.

BACKGROUND

The processing of biomass into bio-fertilizer and / or green energy or bio-energy is known. Well-known technologies include fermentation, composting, and biomass combustion.

Fermentation converts biomass, including wet biomass, into biogas that can be further burned in a gas engine or gas turbine, for example, to generate (green) electricity (and heat). The material that remains after fermentation (i.e. the digestate) can be spread on land, or traded as fertilizer after further treatment such as drying. The wet digestate is rich in nutrients. For example, nitrogen (N) present in the biomass will be converted to ammonium (NH4 +) during the fermentation process. However, part of the nitrogen will disappear from the digestate, and upon drying, by evaporation in the form of ammonia (NH3). The wet digestate is also not practically transportable.

Composting occurs through the activity of bacteria and fungi that are naturally present on the organic material (i.e. the biomass). They break down the material and use the products for their own life processes. Nitrogen (N) is converted into nitrate (NO3), a form of mineral nitrogen that leaches easily.

The thermal energy that is released during the burning of biomass can be used as such, for example for heating, or further converted into (green) electricity using an Organic Rankine cycle (ORC) turbine or steam turbine. The non-combustible fraction of the biomass is left behind in the incineration plant as ash and can be used as fertilizer. However, this fertilizer is low in nitrogen (N) that leaves the combustion plant with the flue gasses as NOx. Other nutrients can also leave the incineration plant with the flue gases as fly ash, unless these nutrients are collected with, for example, a dust filter. Incineration is not energetically favorable for the processing of wet biomass, which must first be dried before burning.

SUMMARY

The method according to the invention solves one or more problems with known technologies for biomass processing by integrating these known technologies in a synergistic manner.

In particular, the present invention provides a method for processing biomass comprising fermentation, drying, and combustion of the biomass, wherein nutrients, in particular nitrogen (N), are separated from the biomass before the combustion process, and after the combustion can be added to the ash, which results in a bio-fertilizer with increased nutrient recovery from the biomass.

Moreover, the (green) energy production, in particular the production of (green) electricity, can be increased with the method according to the invention. If the thermal energy released during incineration is used for (green) electricity production, this results in a higher efficiency compared to the direct burning of the biomass because the biomass is dried before incineration. In this way, wet biomass can also be efficiently burned and thereby generate green electricity. The efficiency can be further increased by converting the biogas that is formed during the fermentation of the biomass into electricity and residual heat, and then converting this residual heat together with the thermal energy from the combustion process into electricity and residual heat.

Consequently, the present invention provides a method for processing biomass, for the production of (green) energy, in particular (green) electricity, and / or for the production of bio-fertilizer comprising the following steps: a) fermentation of a first part of the biomass into biogas and digestate; b) separating the digestate obtained in step a) into a liquid fraction and a solid fraction; c) separating nutrients, preferably nitrogen (N), from the liquid fraction of the digestate obtained in step b) and adding said nutrients to the ash obtained in step e); d) drying the solid fraction of the digestate obtained in step b); and e) burning the dried solid fraction of the digestate obtained in step d) to ash.

In certain embodiments, the method further comprises the following steps: f) converting the biogas obtained in step a) into electricity and residual heat; g) converting the heat released during the combustion in step e) and the residual heat obtained in step f) into electricity and residual heat.

Also provided herein is a bio-fertilizer comprising the ash obtainable by the method according to the invention, as well as the use of the ash obtainable by the method according to the invention as bio-fertilizer.

Further provided herein is a system (100) for processing biomass according to the method according to the invention comprising: - a fermentation unit (10); - a separation unit (12) for separating digestate into a solid and a liquid fraction; - a unit (14) for separating nutrients, preferably nitrogen (N), from the liquid fraction of the digestate, preferably an evaporation unit or a steam stripper; - a drying unit (20); and - a combustion unit (30).

In certain embodiments, the system further comprises: - a unit (40) for converting biogas into electricity and residual heat; and - a unit (50) for converting the residual heat obtained from the conversion of biogas into electricity, and thermal energy obtained from the combustion, into electricity and residual heat.

DESCRIPTION OF THE FIGURES

Figure 1 shows a schematic representation of a system (100) and method according to the invention.

DETAILED DESCRIPTION

Before describing the current methods and systems of the invention, it is to be understood that this invention is not limited to certain methods and systems described, since such methods and systems may, of course, vary. It should also be understood that the terminology used is not intended to be limiting here, since the scope of the present invention will only be limited by the claims in question.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as generally understood by one skilled in the art to which this invention belongs. Although all methods and materials that are the same or equivalent to those described herein can be used in practice or in testing the present invention, the preferred methods and materials are now described. in this description and in the appended claims, the singular forms include "a," "an," and "the" plural forms unless the context clearly dictates otherwise.

The terms "comprising", "includes" and "include" as used herein are synonymous with "including", or "containing", "contains" and "contain", and include or open and close added, non-recited members , elements or process steps.

The terms "comprising", "includes" and "include" also include the term "consisting of".

The present invention provides a method for the processing of biomass, for the production of bio-fertilizer, and / or for the production of (green) energy, in particular (green) electricity, comprising the following steps: a) fermenting a first part of the biomass into biogas and digestate; b) separating the digestate obtained in step a) into a liquid fraction and a solid fraction; c) separating nutrients, preferably nitrogen (N), from the liquid fraction of the digestate obtained in step b) and adding said nutrients to the ash obtained in step e); d) drying the solid fraction of the digestate obtained in step b); and e) burning the dried solid fraction of the digestate obtained in step d) to ash.

Nitrogen (N) present in the biomass, in particular the first part of the biomass, is converted during the fermentation into ammonium (NH4 +), which is recovered before combustion, and then added to the ash obtained in step e) is becoming. In this way, more nitrogen can be recovered from the biomass in the bio-fertilizer compared to incineration alone. In addition, the nitrogen in the bio-fertilizer will consist of ammonium sulfate ((NH 4) 2 SO 4), which leaches less than, for example, nitrate (NO 3) formed during composting.

In further embodiments, nutrients, preferably nitrogen (N), are separated from the vapors released during drying in step d) and added to the ash obtained in step e). This nutrient recovery will result in a further increase in the share of nutrients from the biomass in the bio-fertilizer.

In certain embodiments, the method according to the invention further comprises the following steps: f) converting the biogas obtained in step a) into electricity and residual heat; g) converting the heat released during the combustion in step e) and the residual heat obtained in step f) into electricity and residual heat.

Such an integrated method makes it possible to synergistically produce more electricity, in particular green electricity, from the biomass in comparison with the individual processes.

The term "biomass" as used herein refers to the biodegradable fraction of products, waste and residues from agriculture (including manure flows), forestry and related industries, as well as the biodegradable fraction of industrial and household waste.

In a preferred embodiment, secondary biomass is processed. The term "secondary biomass" as used herein means biomass that cannot be used for human consumption.

The different process steps are discussed in more detail below. Preferably, the various process steps take place on one site, but also methods in which one or more process steps take place on another site are provided.

Veraistinq or anaerobic digestion

The terms "fermentation", "fermentation" and "anaerobic digestion" are used herein as synonyms and refer to a microbial process that takes place in the absence of oxygen (ie anaerobic) in which biomass is converted into simpler products such as methane (CH4), carbon dioxide ( CO2), H2 and H2 O.

Fermentation typically takes place at temperatures between 15 ° C and 60 ° C, depending on the microorganisms responsible for the fermentation. The microorganisms used in a fermentation process are typically a combination of different types of microorganisms, such as thermophilic or mesophilic bacteria.

Another parameter for the fermentation process is the ammonia concentration in the reactor. The ammonia concentration in the reactor is preferably less than 5 g / L. In order to obtain an optimum ammonia concentration in the reactor, the biomass, in particular the first part of the biomass, can be diluted with water, for example water after condensation of the water vapor that is released when evaporating the liquid fraction of the digestate.

The fermentation typically takes place in a fermentation unit or fermentation plant (10). This can take various forms such as a concrete or metal tank reactor or an egg-shaped reactor. Furthermore, a distinction can be made between continuously fed, semi-continuously fed or batch reactors. With continuously fed reactors, mixing of fresh and old material in the fermentation unit is necessary so that the micro-organisms come into contact with the freshly fed material more quickly. Mixing can be done, for example, by stirring, by circulating the liquid, or by introducing biogas.

The wet end product of a fermentation is called "digestate". The composition of the digestate mainly depends on the composition of the biomass that is fermented.

In addition, biogas is also formed in the fermentation plant. The term "biogas" is understood herein to mean the gas mixture that arises after fermentation of the biomass, in particular the first part of the biomass. The biogas primarily comprises methane (CH 4) and carbon dioxide (CO 2), and may further comprise one or more components selected from the group consisting of H 2 O, H 2 S, and NH 3. The exact composition of the biogas is determined by the composition of the biomass that is fermented.

In embodiments of the method according to the invention, the biomass, in particular the first part of the biomass, is pre-treated before fermentation. Possible pre-treatment techniques are shrinking, dry separation, wet separation, pasteurization, sterilization, acidification, etc. Usually the pre-treatment consists of a combination of two or more of the listed techniques.

The biomass that is fermented in the method according to the invention is referred to as a first part of the biomass. The first part of the biomass can be either wet (i.e., liquid) or dry (i.e., solid). The first part of the biomass preferably comprises nitrogen-rich biomass, ie biomass with a nitrogen content comprised between 1% and 20%, preferably between 2% and 20%, still preferably between 5% and 20%, based on the total weight of the biomass. Non-limiting examples of biomass that can be used as the first part of the biomass in the method according to the invention are product streams such as maize, glycerin, soap stock, etc .; waste streams such as fat, flotate, food, fine greenery, etc .; and manure flows from, for example, chicken, pork and beef.

Separation of the diate state

In a separation unit (12), the solid and liquid fraction of the digestate are separated from each other. Examples of a separation unit (12) are a centrifuge or a jack press. Use is preferably made of a centrifuge.

Depending on the biomass that is fermented and the fermentation process itself, the digestibility of the digestate will be easier or more difficult. Fiber-rich material can greatly increase the de-waterability. Use can also be made of chemicals such as polyelectrolites or polymers for better dewatering.

The term "solid or thick fraction of the digestate" is understood herein to mean the fraction of the digestate after separation that comprises at least 15% (w / w) dry matter.

The term "liquid or thin fraction of the digestate" is understood herein to mean the fraction of the digestate after separation that comprises at most 5% (w / w) dry matter.

Not all nutrients distribute evenly between the two fractions after separation: phosphorus goes for the most part to the solid fraction and nitrogen is largely found in the liquid fraction.

In certain embodiments, the method of the invention may further comprise a hygienization step of the digestate before or after separation.

Conversion of biogas into electricity and residual heat

The biogas can be further converted into electricity, heat, mechanical energy or a combination of two or more of these. In addition, there are also possibilities to valorise the biogas, optionally after purification into biomethane, as natural gas of renewable origin. The bio-natural gas can, for example, be injected into the natural gas network, or it can be compressed at a pressure of 200 to 250 bar into bio-CNG (compressed natural gas) or even better referred to as CBG (compressed biogas) as well as CBM (compressed biomethane). Biomethane, preferably purified biomethane, can also be used as a precursor for the production of biopolymers.

In one embodiment, the biogas is converted into electricity and residual heat. Since the electricity comes from biomass, it can be referred to as "green electricity". In certain embodiments, the biogas is converted to electricity and residual heat using a gas turbine. The biogas is burned in the combustion chamber of a gas turbine. The turbine then drives a generator that generates electricity. Biogas can also be converted into electricity and residual heat using a gas engine or fuel cell.

Recovery of nutrients from the liquid fraction of the diestate

In the methods according to the invention, nutrients, in particular nitrogen (N), phosphorus (P) and potassium (K), are recovered to the maximum from the biomass. To this end, nutrients are isolated from the liquid fraction of the digestate. These can be used as such as fertilizer, but are preferably mixed with the ash from the incineration in step e) of the method according to the invention.

In certain embodiments, nutrients from the liquid fraction of the digestate are separated by evaporation. The non-volatile nutrients present in the liquid fraction are concentrated by evaporating the water. The residual heat from step g) of the method according to the invention is preferably used for this evaporation process.

To isolate nitrogen (N) from the liquid fraction by evaporation, a strong acid solution, in particular a sulfuric acid solution, may be added to the liquid fraction prior to evaporation. This is because nitrogen (N) is converted to the volatile ammonia (NH3) in the fermentation process, which disappears in the vapor phase when the liquid fraction is evaporated. By adding the sulfuric acid solution (H 2 SO 4), ammonium sulfate (NH 4 SO 4) is formed which, together with the other non-volatile nutrients, remains behind during evaporation. Alternatively, the sulfuric acid (H 2 SO 4) may be added to the vapor phase after evaporation (i.e., acid gas or air washing) to produce ammonium sulfate (NH 4 SO 4).

An alternative technology for isolating nutrients from the liquid fraction of the digestate is steam stripping, where steam is blown through the liquid fraction so that volatile nutrients (for example ammonia (NH3)) end up in the gas phase. After purification of the gas phase, for example using an acid gas or air washer with sulfuric acid (H 2 SO 4), nitrogen (N) in the form of ammonium sulphate (NH 4 SO 4) can thus be separated from the liquid fraction of the digestate.

Also provided herein are methods in which the nutrients are separated from the digestate prior to separation into the solid and liquid fraction. To this end, the same technologies can be used as described above, subject to the necessary adjustments.

Drying

The solid fraction of the digestate is dried in the methods of the invention.

In one embodiment, the residual heat from step g) of the method according to the invention is used for the drying process.

In other embodiments, biothermal is dried. By "biothermal drying" it is understood that the heat for the drying process is biological heat, namely heat from micro-organisms that break down the biomass in the presence of oxygen.

In a preferred embodiment, biothermal drying is performed for up to 10 days. In an even more preferred embodiment, biothermal drying is performed for up to 8 days. In a most preferred embodiment, biothermal drying is performed for a maximum of 7 days (i.e., a maximum of 1 week).

Microorganisms are naturally present in biomass. The digestate, however, mainly comprises anaerobic microorganisms from the fermentation process. These anaerobic microorganisms will not grow under the aerobic conditions during drying. Therefore, a second part of the biomass will have to be mixed with the solid fraction of the digestate before it can be dried biothermally. This second part of the biomass preferably comprises biomass with a dry matter content comprised between 25% and 100%, such as between 25% and 50%, based on the total weight of the biomass. Preferably, this second part of the biomass is low in nitrogen, i.e., biomass with a nitrogen content of less than 2%, preferably less than 1%, such as about 0.5%, based on the total weight of the biomass. Still preferably, the nitrogen (N) is present in the second part of the biomass in the form of ammoniacal compounds. Non-limiting examples of biomass that can be used as the second part of the biomass are waste streams such as pruning, wood waste, coarse greenery, champost, etc. and manure streams from, for example, chicken, pig, and beef.

In embodiments of the method according to the invention, the second part of the biomass is pretreated, for example reduced or crushed, before mixing with the solid fraction of the digestate.

In still other embodiments, biothermal drying is performed and use is made of the residual heat from step g) of the method according to the invention. In this embodiment too, the solid fraction of the digestate will first have to be mixed with a second part of the biomass before drying.

The drying process typically takes place in a drying unit or drying installation (20). Any type of drying unit can be used, such as a tunnel dryer, a fluid bed dryer, a drum dryer, etc. Preferably, use is made of a tunnel dryer, with which both biothermic drying and with (residual heat) can be used.

Recovery of volatile nutrients from the drying process

Upon drying the solid fraction of the digestate, optionally mixed with a second part of the biomass, volatile nutrients, in particular nitrogen (N) in the form of ammonia (NH3), will disappear with the vapors.

In certain embodiments, nutrients, preferably nitrogen (N), are separated from the vapors released during drying in step d). These nutrients are then preferably added to the ash obtained in step e) of the method according to the invention.

The separation of nutrients, preferably nitrogen (N), from the vapors released during the drying process can be done via gas washing, whereby the vapors are sent through a gas washer (24) with a liquid (i.e. the washing water). For the separation of ammonia (NH3) from the vapors, use is preferably made of an acid scrubber comprising a sulfuric acid solution, which ammonia (NH3) separates from the vapors via a chemical reaction to form ammonium sulfate (NH4 SO4).

The nutrients can then be separated from the wash water using the technologies described above for separating nutrients from the liquid fraction of the digestate.

The washing water can also be mixed with the liquid fraction of the digestate obtained in step b) of the method according to the invention, from which the nutrients can be isolated as described above.

Combustion

In step e) of the method according to the invention, the dried solid fraction of the digestate, and optionally the dried second part of the biomass, is burned to ash, flue gases comprising CO 2 and H 2 O, and thermal energy, in a combustion unit (30).

In certain embodiments, the dried solid fraction of the digestate, optionally mixed with the dried second part of the biomass, is separated according to size in a separation unit (22), for example a sieve. The coarse fraction (ie fraction with a (grain) g root of at least 50 mm, preferably a (grain) size included between 50 mm and 100 mm) can then be mixed with the solid fraction of the digestate and optionally a second part of the biomass for further drying. The fine fraction (i.e. fraction with a (grain) size of at most 50, preferably a (grain) size comprised between 20 mm and 50 mm) can be burned.

The ashes comprise the non-combustible fraction of the dried solid fraction of the digestate, optionally mixed with the dried second part of the biomass, and can be used as fertilizer. Since the ashes come entirely from biomass, the fertilizer can be referred to as "bio-fertilizer".

Typically, the ash comprises bottom ash, which remains in the combustion unit (30), and fly ash, which disappears together with the flue gases. In certain embodiments, the fly ash is collected and added to the bottom ash. This allows the nutrient recovery from the biomass to be further increased. Collecting the fly ash can be done with a Dust filter (32).

Moreover, when the nutrients, in particular nitrogen (N), which are separated from the liquid fraction of the digestate and / or from the vapors released during drying, are added to the ash, in particular the bottom ash, a bio-fertilizer obtained with increased nutrient recovery. In certain embodiments, at least 80%, such as at least 85%, preferably at least 90%, such as at least 91%, 92%, 93%, 94%, or 95%, of the nutrients from the biomass are recycled into the bio-fertilizer.

In certain embodiments, the flue gases are purified, for example by gas washing. In this way the emission of harmful substances into the environment is limited.

In certain embodiments, the dried solid fraction of the digestate, optionally mixed with the dried second part of the biomass, can be mixed with a third part of the biomass, before burning. This third part of the biomass is preferably combustible biomass. Non-limiting examples of combustible biomass are product streams such as straw, wood dust, etc. and waste streams such as wood and sieve overflow.

Conversion of thermal energy into electricity and residual heat

The thermal energy released during the combustion process in step e) of the process according to the invention can be used as such, for example for the heat-demanding processes of the process according to the invention, such as the drying process and the evaporation of the liquid fraction of the process. digestate.

In certain embodiments, the thermal energy is converted into electricity and residual heat. A so-called Rankine cycle is usually used for this conversion. In this cycle, water is heated to steam. The steam then expands over a steam turbine with work being supplied to the turbine. The turbine then drives the electricity generator. When the steam in the Rankine cycle is replaced by an organic fluid such as pentane, hexane, toluene, ammonia, etc., one speaks of the Organic Rankine Cycle (ORC). The advantage of these media is that they evaporate at a lower temperature than steam. Thus, in certain embodiments, the thermal energy is converted to electricity and residual heat using a steam turbine or an ORC turbine, preferably an ORC turbine.

Since the biomass was dried before the combustion process, the efficiency of the (green) electricity production from the biomass is higher than if the biomass were burned directly (i.e. without first drying).

In preferred embodiments of the method according to the invention, the residual heat obtained in step f) together with the thermal energy released during the combustion in step e) is converted into electricity and residual heat. In this way the efficiency of the electricity production from the biomass can be further increased.

CO conversion? in biomass

In certain embodiments, the CO2 released during the combustion in step e) and optionally the combustion of biogas for conversion into electricity and residual heat in step f) is converted into biomass. Algae and bacteria reproduce at lightning speed and only use sunlight, CO2 and nutrients. The algae biomass and / or bacterial biomass thus obtained can be used as biofuel, or as nutrical (e.g. Dunaliella salina because of the high beta-carotene content), or for other applications.

Furthermore, the present invention provides for the use of the ash, in particular the bottom ash, obtained according to the method described herein as a bio-fertilizer.

Also provided herein is bio-fertilizer comprising the ash, in particular the bottom ash, obtained according to the method described herein.

Another aspect of the present invention is a system (100) for the processing of biomass, for the production of bio-fertilizer, and / or for the production of (green) energy, in particular (green) electricity, comprising: - a fermentation unit (10); - a separation unit (12) for separating digestate into a solid and a liquid fraction; - a unit (14) for separating nutrians, preferably nitrogen (N), from the liquid fraction of the digestate, preferably an evaporation unit or a steam stripper; - a drying unit (20); and - a combustion unit (30).

In certain embodiments, the system further comprises: - a unit (40) for converting biogas into electricity and residual heat; and - a unit (50) for converting the residual heat obtained from the conversion of biogas into electricity, and thermal energy obtained from the combustion, into electricity and residual heat. EXAMPLES Example 1

An embodiment of the invention is discussed with reference to Figure 1. A first part of the biomass (101) is mixed with water (113) from the evaporation unit (14) in a mixing unit (6). The mixture is fermented in a fermentation unit (10) into biogas (103) and digestate (105). The wet digestate (105) is separated into a separation unit (12) in a liquid (107) and a solid (109) fraction. The liquid fraction (107) is evaporated in an evaporation unit (14), whereby nutrients (115) such as nitrogen (N), phosphorus (P) and potassium (K) are separated from the liquid fraction. These nutrients are added to the bottom ash (132) from incineration. To isolate nitrogen (N) from the liquid fraction (107) of the digestate, a sulfuric acid solution (111) is added to the liquid fraction (107) of the digestate before evaporation.

The solid fraction (109) of the digestate is mixed in a mixing unit (16) with, among other things, a second part of the biomass (120). This second part of the biomass (120) is broken into a refraction unit (17) before mixing with the solid fraction (109) of the digestate. The mixture is dried in a tunnel dryer (20). The dried mixture (126) is separated into a coarse (128) and a fine (130) fraction in a separation unit (22). The coarse fraction (128) is mixed in the mixing unit (16) with the solid fraction (109) of the digestate, and a second part of the biomass (120) and is further dried. The vapors (122) released during drying are sent through a gas washer (24). The washing water (124) is added to the liquid fraction (107) of the digestate, thus separating nutrients (115) from the vapors (122) released during drying by evaporation.

The fine fraction (130) of the dried mixture is burned in an incinerator (30) to bottom ash (132) and fly ash (134) containing nutrients such as phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg ). Thereby thermal energy (144) is released and flue gases (136). The fly ash (134) leaving the incinerator (30) with the flue gases (136) is collected via a Dust filter (32) and added to the bottom ash (132) remaining in the incinerator (30). This bottom ash (132), to which the fly ash (134), the nutrients (115) from the liquid fraction (107) of the digestate, and the nutrients (115) from the vapors (122) exempted during drying, are added , is used as a bio-fertilizer (142). The CO2 (138) which is released during the combustion process, and which leaves the incinerator (30) via the flue gases (136), is converted into biomass (140) by microorganisms (38). The flue gases (136) are controlled via a gas washer (34) before leaving the system (100).

The biogas (103) that is produced during fermentation is led to a gas turbine (40) for the production of electricity (150) and residual heat (152). The residual heat (152) together with the thermal energy (144) from the combustion process is converted into electricity (148) and residual heat (146) using a steam turbine (50). The residual heat (146) is used in the evaporation unit (14) and the tunnel dryer (20).

An example mass balance (tonnes / year) for the different processes of the above-described method is shown in Table 1. According to this example, 130,000 tonnes of biomass are converted into bio-fertilizer each year comprising 93% of the nutrients of the biomass, in particular, N, P, K, Ca and Mg, and 80,000 MWh of green electricity.

Claims (15)

CONCLUSIONS A method for processing biomass comprising the following steps: a) fermenting a first part of the biomass into biogas and digestate; b) separating the digestate obtained in step a) into a liquid fraction and a solid fraction; c) separating nutrients, preferably nitrogen (N), from the liquid fraction of the digestate obtained in step b) and adding said nutrients to the ash obtained in step e); d) drying the solid fraction of the digestate obtained in step b); and e) burning the dried solid fraction of the digestate obtained in step d) to ash. The method of claim 1, further comprising the steps of: f) converting the biogas obtained in step a) into electricity and residual heat; g) converting the heat released during the combustion in step e) and the residual heat obtained in step f) into electricity and residual heat. Method according to claim 1 or 2, wherein nutrients, preferably nitrogen (N), are separated from the vapors released during drying in step d) and added to the ash obtained in step e). A method according to any one of claims 1 to 3, wherein the nutrients from the liquid fraction of the digestate are separated by evaporation, preferably evaporation using the residual heat obtained in step g). A method according to claim 3, wherein the nutrients from the vapors released during drying in step d) are separated by gas washing of said vapors with washing water, preferably a sulfuric acid solution, and mixing the washing water with the liquid fraction of the digestate obtained in step b). The method according to any of claims 1 to 5, wherein in step d) a second part of the biomass is mixed with the digestate before drying, and wherein the drying takes place biothermically. A method according to any one of claims 1 to 6, wherein the drying in step d) takes place using the residual heat obtained in step g). A method according to any one of claims 1 to 7, wherein the fly ash is collected and added to the bottom ash. A method according to any one of claims 1 to 8, further comprising the step h) converting the CO2 released during the combustion in step e) into biomass, preferably algae biomass and / or bacterial biomass. A method for the production of electricity comprising the method according to any of claims 2 to 9. A method for the production of bio-fertilizer comprising the method according to any of claims 1 to 9, wherein the ash obtained in step e) is the bio-fertilizer. Use of the ash obtainable by the method according to any of claims 1 to 9 as bio-fertilizer. A bio-fertilizer comprising the ash obtainable by the method according to any one of claims 1 to 9. A system (100) for processing biomass according to the method according to one of claims 1 to 9, comprising: - a fermentation unit (10); - a separation unit (12) for separating digestate into a solid and a liquid fraction; - a unit (14) for separating nutrients, preferably nitrogen (N), from the liquid fraction of the digestate, preferably an evaporation unit or a steam stripper; - a drying unit (20); and - a combustion unit (30). The system of claim 14, further comprising: - a unit (40) for converting biogas into electricity and residual heat; and - a unit (50) for converting the residual heat obtained from the conversion of biogas into electricity, and thermal energy obtained from the combustion, into electricity and residual heat.